Having both an AST and SCH shield active would be OP in any instance, as losing one shield still means there is another one right behind it, kinda defeats the need of ever having Regen honestly.

However the only thing that maybe worth adjusting is allowing a shield with the highest HP restored amount to stay active even if another one is tried to be casted over it. For example a crit Galvanize shouldn't get overwritten by a weaker AST shield, so the stronger one should stay active until it is destroyed. I suppose this would be rare as any SCH/AST pair shouldn't be overwriting each others shields but it can still be annoying, especially in 24man situations when someone from another alliance gets involved and sabotages your buffs.